This invention relates to a control system adapted to control an air circulation fan of a temperature conditioning apparatus as a function of the difference in temperature between the apparatus plenum temperature and the return air temperature. This may include heating and/or cooling apparatus, however; to simplify the description of the invention a forced air furnace is specifically described. In a specific embodiment then, this invention relates to a control system adapted for a forced warm air furnace air circulation fan or blower control and especially to maintaining furnace system efficiency during night setback operation. The setting of a thermostat to a lower temperature control point during the night (i.e. night setback) saves fuel because it reduces the building load imposed on the heating system. The resulting efficiency of forced warm air furnace systems may be lowered considerably, however, because of the relationship between the fixed steady state plenum temperature rise and make-break circulating fan switch temperature settings on one hand, and the variable room air temperatures (combustion and return air temperatures) on the other hand.
The improved control system to avoid deterioration of furnace system efficiency during night setback is to reduce the air circulating fan turn-off setpoint by substantially the same amount in degrees that the room temperature is dropped, and in the preferred embodiment shown, this is by adding a return air temperature sensor in addition to the plenum temperature sensor and feeding both signals to a circuit which responds to the difference in the two signals.